Method for the production of single crystalline tio2 flakes

ABSTRACT

The present invention is related to a method for the production of single crystalline TiO 2  flakes in the rutile crystal structure, to single crystalline TiO 2  flakes obtained by this method as well as to the use thereof, especially as pigments in several application media.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method for the production of singlecrystalline TiO₂ flakes, to single crystalline TiO₂ flakes produced bythis method as well as to the use thereof, in particular as pigments.

2. Description of the Related Art

Titanium dioxide (TiO₂) is widely used in various fields such aspigments, photo catalysts, solar cells, UV absorbents and waterrepellants, and various methods have been developed in order to findappropriate methods for producing TiO₂, especially in its rutile crystalform.

For example, Japanese Unexamined Patent Application Publication No.58-88121 (Patent Document 1) discloses a method to obtain titaniumdioxide particles of the rutile type which are grown in a specificcrystal axis direction, wherein potassium titanate (K₂O.nTiO₂) fibersobtained by a flux method are treated with an acid resulting inneedle-shaped water containing TiO₂-particles which are, in thefollowing, heat treated in order to give rutile TiO₂-needles of a verysmall particle size.

Japanese Unexamined Patent Application Publication No. 04-144918 (PatentDocument 2) discloses a method to obtain flaky titanium dioxideparticles by dissolving titanium alkoxide and an organic alkalinesubstance in an organic solvent and forming a film on a belt by coatingthis solution on a smooth surface, followed by hydrolysis, drying,peeling, and calcination steps.

Japanese Unexamined Patent Application Publication No. 07-157312 (PatentDocument 3) discloses a method for obtaining flaky titanium dioxide byacid treatment of potassium di-titanate fibers obtained by a meltingmethod in the presence of a fluxing agent, whereby by means of the acidtreatment all of the K+ ions within the crystals are eluted, followed byheat treating the resulting particles to obtain a flake type powder.

However, in the techniques described in these Patent Documents, theobtained particle shape of the titanium dioxide particles may partly notbe suitable as pigment, in particular if larger flaky shaped crystalsare needed, or the production method should be further improved,especially with respect to energy efficiency, production time and cost.

For example, the titanium dioxide particles obtained by the methoddescribed in the Patent Document 1 exhibit a whisker type shape havingan extremely fine size (particle diameter of at most 6 μm). Therefore,the use of these particles as pigments is limited, especially withregard to their potential use as highly reflective interferencepigments.

Further, although the titanium dioxide particles obtained by the methoddescribed in the Patent Document 2 have a flake type shape, they exhibita polycrystalline structure and the production method is based onorganic compounds and solvents which cause high cost and technologicalefforts and, thus, more complications.

Further, the titanium dioxide particles obtained by the method describedin Patent Document 3 exhibit a flake type shape of an appropriate size,but the requirement of two calcination steps takes time and causes highcost and energetic waste in the production procedure.

Thus, the aim of the present invention is to reduce energy cost of theproduction step and to offer a method to effectively produce singlecrystalline TiO₂ flakes, preferably from inorganic compounds, which aresuitable for use as pigments having luster, high refractive index aswell as large particle size. A further aim of the present invention isto provide single crystalline TiO₂ flakes produced according to saidprocess, as well as to suggest the use thereof.

SUMMARY OF THE INVENTION

The inventors have found that the problems described above can be solvedand that single crystalline TiO₂ flakes suitable for use as pigments canbe effectively produced by mixing a phosphorus compound with a titaniumcompound in order to form a titanium dioxide precursor, and then bycalcinating the obtained precursor, and completed the present invention.

The present invention is following.

(1) A method for the production of single crystalline TiO₂ flakes by thefollowing steps:

-   -   1. mixing at least a titanium compound and a phosphorous        compound whereby a titanium dioxide precursor is formed; and    -   2. calcining the titanium dioxide precursor at a temperature in        the range of from 800° C. to 1400° C. as a single calcination        step.

(2) The method according to (1), wherein the mixing is executed in anaqueous medium.

(3) The method according to any one of (1) to (2), wherein a fluxingagent is present in the mixing step.

(4) The method according to (3), wherein the fluxing agent is a compoundselected from one or more of Na₂SO₄, K₂SO₄, NaCl and KCl.

(5) The method according to any one of (2) to (4), wherein the methodcomprises drying of the titanium dioxide precursor prior to thecalcination step.

(6) The method according to any one of (1) to (5), wherein the calciningis executed in an oxygen containing atmosphere.

(7) The method according to any one of (3) to (6), wherein a productobtained in the calcination step according to (3) is treated with hotwater.

(8) The method according to any one of (1) to (7), wherein the titaniumcontaining compound is a compound selected from one or more of titaniumtetrachloride, titanyl sulfate, titanium sulfate and titaniumtrichloride.

(9) The method according to any one of (1) to (8), wherein thephosphorous compound is a compound selected from one or more oftrisodium phosphate, phosphorous pentoxide, phosphoric acid, phosphorousacid and tripotassium phosphate.

(10) Single crystalline TiO₂ flakes obtained by the method according toany one of (1) to (9).

(11) Single crystalline TiO₂ flakes according to (10), having a rutilecrystal structure.

(12) Single crystalline TiO₂ flakes according to (10) or (11), having aparticle diameter in the range of 10 to 200 μm, according to anequivalent of the corresponding circle diameter.

(13) Single crystalline TiO₂ flakes according to any one of (10) to(12), having the shape of a letter “V” when viewed at the largestsurface of the flakes.

(14) Use of single crystalline TiO₂ flakes according to any one of (10)to (13) in a paint, ink, coating composition, plastic or cosmetic.

(15) Use according to (14) as white pigment, photo catalyst, hostmaterial for dye sensitized solar cells, UV absorbent or waterrepellent.

According to the present invention, a method to effectively producesingle crystalline TiO₂ flakes which are suitable for use as pigmentscan be offered.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the method for the production of single crystalline TiO₂flakes according to the present invention and the single crystallineTiO₂ flakes obtained by this method will be described in detail.However, the present invention is not limited to these particularembodiments as long as the intention of the present invention isfollowed.

The method for the production of single crystalline TiO₂ flakesaccording to a first embodiment of the present invention (hereinafter,abbreviated to “the production method of the present invention”)contains: “a step of mixing at least a titanium compound and aphosphorous compound whereby a titanium dioxide precursor is formed”;and “a step of calcining the titanium dioxide precursor at a temperaturein the range from 800° C. to 1400° C. as a single calcination step”.

The inventors have found that single crystalline TiO₂ flakes suitablefor use as pigments can be effectively produced by mixing a phosphoruscompound with a titanium compound in order to form a titanium dioxideprecursor and then by calcination of the obtained precursor in a singlecalcination step. The single crystalline TiO₂ flakes have a highrefractive index and high luster, therefore, they are particularlysuitable as highly reflective pigments, especially interferencepigments. The production method of the present invention enables aneffective production of good quality single crystalline TiO₂ flakes withthe need for only a single calcination step. Therefore, this method ishighly suitable for the suppression of production cost and efforts.Furthermore, it can produce single crystalline TiO₂ flakes, in whichtwin crystals and coagulations hardly occur and in which crystallinityis high and diameter control is possible.

Therefore, single crystalline TiO₂ flakes in the rutile form having anextremely high refractive index can be produced.

Although the detailed mechanisms for the formation of single crystallineTiO₂ flakes according to the present invention have not beensufficiently clarified, the inventors have confirmed that TiO₂ flakeshave been formed with a titanium compound under the co-existence ofphosphorus ions. Hereinafter, the production method of the presentinvention will be described in detail.

The production method of the present invention contains as the firststep a step of mixing at least a titanium compound and a phosphorouscompound whereby a titanium dioxide precursor is formed (hereinafter,abbreviated to the “mixing step”).

In the first and most simple embodiment of the present invention, theconcrete types for the titanium compound and for the phosphorus compoundas well as a concrete method for mixing are not limited to any specificsas long as a titanium dioxide precursor can be formed. Since theformation of a titanium dioxide precursor will occur even if puretitanium is used, the titanium dioxide precursor is formed independentlyof the kind of starting materials for the titanium compound and thephosphorous compound, respectively. Here, the “titanium dioxideprecursor” is considered to contain titanium oxide hydrate as the mayorcomponent.

As a second step, the production method according to the presentinvention contains a step of calcining the titanium dioxide precursor ata temperature in the range of from 800° C. to 1400° C. as a singlecalcination step (hereinafter, abbreviated to the “calcination step”).

Anatase type TiO₂ is transformed into the rutile type by calcination ofa titanium oxid precursor at a temperature of at least 800° C. Sincerutile TiO₂ flakes are preferred according to the present invention, theminimum calcination temperature in the present calcination step is 800°C. Calcination of a titanium dioxide precursor at a temperature of morethan 1400° C. would result in a bar type single crystal TiO₂, ratherthan in the flake type. Therefore, the calcination temperature in thepresent production method is usually at least 800° C., and morepreferably at least 900° C. Also it is usually at most 1400° C. and morepreferably at most 1250° C., leading to a preferred temperature range offrom 900° C. to 1250° C.

The calcination time should be appropriately selected according to thedesired shape of the single crystalline TiO₂ flakes (particle diameter,thickness, and aspect ratio). It is usually at least 5 minutes,preferably at least 10 minutes, and more preferably at least 2 hours.Further, this value is usually at most 12 hours, preferably at most 10hours, and more preferably at most 5 hours.

The calcination atmosphere is not limited to any specifics as long asTiO₂ can be formed. However, in order to reliably produce the oxide, thecalcination step is preferably carried out under oxygen containingatmosphere.

The concrete method for mixing of the titanium compound and phosphoruscompound at the mixing step is not limited to any specifics, however,they are preferably mixed in an aqueous medium because working inaqueous solution is comfortable and easy to handle. If the mixing stepaccording to the present invention is executed in an aqueous medium,this variation of the present process represents the second embodimentof the present invention.

The addition of the phosphorus compound to the aqueous medium may becarried out prior to the addition of the titanium compound, at the sametime as the addition of the titanium compound, or after the addition ofthe titanium compound. However, when the phosphorus compound is added tothe aqueous medium prior to the addition of the titanium compound, theyield of the TiO₂ flakes may be increased. Therefore, the lattersequence is preferred.

When the mixing is carried out within an aqueous medium, it is preferredto hold the aqueous solution at a pH value in the region of from pH 6 to8 in order to avoid damage to the crucible by gas generated duringcalcining. For example, when an acidic titanium compound such astitanium tetrachloride is added to the aqueous solution containing abasic phosphorus compound such as trisodium phosphate, this solution isneutralized fully or at least to a certain degree.

Further, the pH value may be adjusted to pH 6 to 8 by separately addingan acidic solution such as hydrochloric acid or sulfuric acid or a basicsolution such as sodium hydroxide aqueous solution or sodium carbonate.Furthermore, it is desirable for the addition of the titanium compoundto be carried out gradually over a longer time period in order tosuppress a rapid pH variation.

According to the present invention, the “aqueous medium” indicates amedium mainly comprising tap water or purified water and it may also bean aqueous solution comprising another component in addition oralternatively to the pure water such as deionized water.

According to a third embodiment of the present invention, it ispreferred that a fluxing agent is added to the above mentioned startingmaterials prior to the calcination step. Here, the “fluxing agent”indicates a type of compound, in particular a metal salt, that functionsas a dissolution means in a so called “fluxing growth process”, whichleads to the formation of TiO₂ from the titanium dioxide precursor atthe calcination step.

For being useful as a fluxing agent in the production process accordingto the present invention, the corresponding metal salts should exhibit amelting temperature which is not less than 800° C. but does not achieveor exceed the melting temperature of the resulting TiO₂. In addition,they have to be water soluble. Examples are metal salts such as Na₂SO₄,K₂SO₄, NaCl and KCl, either alone or in combination of two or morethereof, whereby KCl may only be used in combination with at least oneof the other salts mentioned. From the view point of cost and readyavailability, Na₂SO₄ and K₂SO₄ are preferred, and Na₂SO₄ is particularlypreferred.

Since the addition of a fluxing agent influences the crystal growthprocess of the resulting pigments, the usage amount of the fluxing agentshould be appropriately selected according to the desired shape of thesingle crystal TiO₂ flakes (particle diameter, thickness, and aspectratio). The ratio of the fluxing agent mol number against the Ti atommol number in the used titanium compound (fluxing agent mol number/Tiatom mol number in the titanium compound), it is usually at least 1.0,and more preferably at least 3.0. Further, this value is usually at most30, and more preferably at most 10. In case of a ratio of less than 1.0,the fluxing salt treatment effect would be insufficient. On the otherhand, if the ratio would exceed the value of 30, no further improvementin the flake formation would occur and a larger scale washing step toeliminate them later would be necessary.

The addition of the fluxing agent into the mixture of the startingmaterials may be executed prior to the addition of the titaniumcompound, at the same time as the addition of the titanium compound, orafter the addition of the titanium compound. However, when the fluxingagent is added after the addition of the titanium compound, the yield ofthe TiO₂ flakes may be increased. Therefore, the latter adding sequenceis particularly preferred.

The production method according to the present invention may preferablyalso contain other steps, in addition to the mixing step and calcinationstep. This belongs to the following steps:

-   -   drying step of the titanium dioxide precursor, in order to        obtain a powder of the molten salt mixture prior to the        calcination step (hereinafter, abbreviated to “drying step 1”);        The temperature in drying step 1 is preferably in the range of        from 70° C. to 180° C., although the method is not limited to        this.    -   in order to remove impurities such as chloride, sulfate, etc.        from the product obtained in the calcination step, a step to        treat the product obtained at the calcination step with warm        water (hereinafter, abbreviated to “washing step”); Warm water        used at the washing step is not limited to any specifics,        however, purified water and deionized water are preferred. Here,        the temperature of the warm water is preferably in the range of        from 40° C. to 100° C. In case that a fluxing agent is used for        the present production method, such a washing step is        indispensable.    -   in order to separate the TiO₂ flakes from the solution        containing dissolved impurities after the washing step, a step        to filter and separate the undissolved solids (the TiO₂ flakes)        after the washing step (hereinafter, abbreviated to “filtration        step”); and    -   in order to dry the TiO₂ flakes, a step to dry the TiO₂ flakes        (hereinafter, abbreviated to “drying step 2”). This drying step        is executed preferably in a temperature range of from 20° C. to        180° C., although the present process is not limited thereto.

Regarding the titanium compound which may be used as a starting materialfor the present production method, water soluble titanium compounds arepreferred, in particular in case the process is executed in an aqueousmedium. Organic titanium compounds as well as inorganic titaniumcompounds may be used, but inorganic titanium compounds are clearlypreferred. Using water soluble inorganic titanium compounds makes theproduction process simple and easy to handle, in combination with noneed for expensive apparatuses and explosion protection.

Inorganic titanium compounds are preferably inorganic titanium salts.Examples of titanium salts are: titanium tetrachloride, titanium oxysulfate, titanium sulfate and titanium tri-chloride. However, from theview point of the cost and ready availability, titanium tetra-chlorideand titanium sulfate are preferred. Here, the type of the used titaniumcompound is not limited to one type and at least two types may be usedtogether.

The usage amount of the titanium compound should be appropriatelyselected according to the desired amount of the single crystalline TiO₂flakes to be produced. As the ratio of Ti atom mol number in the singlecrystalline TiO₂ flakes against the mol number in the used titaniumcompound (Ti atom mol number in the single crystalline TiO₂ flakes/Tiatom mol number in the titanium compound), it is usually at least 0.3,preferably at least 0.4 and more preferably at least 0.5. Further, thisvalue is usually at most 0.9, preferably at most 0.95 and morepreferably at most 1.0.

As the phosphorus compound, phosphate compounds such as phosphoric acid,phosphates, condensed phosphoric acid, and condensed phosphate may belisted, and any one can be used as long as it is water soluble. Amongthem, from the view point of cost and ready availability, trisodiumphosphate, phosphorus pentoxide, phosphoric acid, phosphorous acid, andtri-potassium phosphate are preferred. Trisodium phosphate isparticularly preferred. Here, the types of the used phosphorus compoundis not limited to one type and at least two types may be used together.

The usage amount of the phosphorus compound should be appropriatelyselected according to the desired shape of the single crystal TiO₂flakes (particle diameter, thickness, and aspect ratio). As the ratio ofphosphorus compound mol number against the Ti atom mol number in theused titanium compound (phosphorus compound mol number/Ti atom molnumber in the titanium compound), it is usually at least 0.01,preferably at least 0.1, and more preferably at least 1.0. Further, thisvalue is usually at most 10, preferably at most 7.0, and more preferablyat most 5.0.

According to the present production process, single crystalline TiO₂flakes of high quality may be effectively produced. Thus, a furtherobject of the present invention is single crystalline TiO₂ flakesproduced by the process mentioned above.

The single crystalline TiO₂ flakes according to the present inventionusually exhibit the shapes and sizes as described below, although theyare not limited thereto.

The average particle diameter of the TiO₂ flakes is usually at least 10μm, preferably at least 15 μm. And this value is usually at most 200 μm,preferably at most 150 μm.

As soon as the average particle diameter of the TiO₂ flakes is withinthe region mentioned above, it is possible to obtain a pigment havinghigh luster. Here, the “average particle diameter” expresses a diameterof a circle corresponding to the largest length or width of the flake,and indicates the average value obtained from the particle sizedistribution based on the volume.

The thickness of the flakes is usually at least 0.1 μm, preferably atleast 0.2 μm. And this value is usually at most 2.0 μm, preferably atmost 1.0 μm. As long as the thickness is within this region, pearlluster of the resulting TiO₂ flakes may occur, optionally in combinationwith interference colour, depending on the actual thickness of theflakes.

The aspect ratio of the flakes is usually at least 5, preferably atleast 7.5. This value is usually at most 150, preferably at most 100. Ahigh aspect ratio leads to good orientation and thus, high luster, ofthe resulting TiO₂ flakes in any coating layer to which they might beadded.

The crystal structure of the TiO₂ flakes according to the presentinvention is preferably the rutile type. Rutile TiO₂ is known to exhibita high refractive index and, since the TiO₂ particles exhibit a flakyshape, also to exhibit high luster.

By means of the production method according to the present invention,for example, particles as shown in FIGS. 1 (a) and (b) are obtained. Theparticles are separated by a border line as shown in FIG. 1 (c). Afterbeing physically separated from each other, the single crystalline TiO₂flakes have a shape as shown in FIG. 2 (a) “V letter” type or (b)“triangle” type. The single crystalline TiO₂ flakes have a “V”—letterlike shape and “triangle” like shape when the largest surface of theflake is observed, as shown in FIGS. 3 and 4. Therefore, the singlecrystalline TiO₂ flakes may exhibit a relatively large surface and,thus, a good orientation in the application medium.

The single crystalline TiO₂ flakes of the present invention are suitableas pigments, particularly for paints, inks, coating compositions,plastics or cosmetics. Therefore, the use of the present singlecrystalline TiO₂ flakes as pigments in paints, inks, coatingcompositions, plastics or cosmetics also one object of the presentinvention. Further, as the single crystalline TiO₂ flakes of the presentinvention have either a higher refractive index and/or better lusterthan traditional TiO₂ particles or flakes, they may also advantageouslybe used as photo catalysts, host material for dye sensitized solarcells, UV absorbents, water repellents and in all other applicationswhere TiO₂ flakes may usually be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM image of the surface of single crystalline TiO₂ flakesobtained by the production method in accordance with an embodiment ofthe present invention.

FIG. 2 is a schematic view showing a formation state of the singlecrystalline TiO₂ flakes (V letter type and triangle type) obtained bythe production method in accordance with an embodiment of the presentinvention.

In this photograph and schematic view, the single crystalline TiO₂flakes of the V letter type (a), the single crystalline TiO₂ flakes ofthe triangle type (b) and a border line between V letter and triangle(c) are shown.

FIG. 3 is a SEM image of the surface of single crystalline TiO₂ flakes(V letter type) obtained by the production method in accordance with anembodiment of the present invention.

FIG. 4 is a SEM image of the surface of single crystalline TiO₂ flakes(triangle type) obtained by the production method in accordance with anembodiment of the present invention.

FIG. 5 is a flowchart showing the production method for singlecrystalline TiO₂ flakes in accordance with a preferred embodiment of thepresent invention.

Hereinafter, the present invention will be described in more detail byreferencing examples, however, the present invention is not limited tothese examples.

Example 1

Into 1600 g of deionized water, 5.6 g of sodium phosphate is added andstirred while heating to 95° C. Into this solution, 200 g of titaniumtetrachloride solution (32.0% concentration) is added over the course oftwo hours while adjusting the pH with sodium hydroxide aqueous solution(32.0% concentration). After adding all of titanium tetrachloridesolution, the pH is raised to 7. Further, 130 g of sodium sulfate isadded under stirring. Afterwards, the resulting solution is dried and,then calcined at 1000° C. for two hours. After cooling, the obtainedproduct is washed within warm water under agitation in order to removechloride and sodium sulfate, and the TiO₂ flakes are obtained.

The obtained TiO₂ flakes have an average particle diameter of 70 μm andmost of them exhibit the V-like shape. From the X-ray diffractionanalysis, the main surface of the flake have the (110) orientation andthe crystal structure is the rutile type. Further, the analysis withelectron microscope confirms that they are single crystals.

The obtained TiO₂ flakes are added into a usual nitro-cellulose lacquerin an amount of 5% by weight, in relation to the total weight of themixture. The resulting coating composition containing the obtained TiO₂flakes is coated onto paper, resulting in a lustreous coating afterdrying. The TiO₂ flakes prepared according to Example 1 present thestrongest luster among the examples.

Example 2

Into 600 g of deionized water, 5.6 g of sodium phosphate is added andstirred while heating to 95° C. Into this solution, 200 g of titaniumsulfate solution (32.0% concentration) is added over the course of twohours while adjusting the pH with sodium hydroxide aqueous solution(32.0% concentration). After adding all of titanium sulfate solution,the pH is raised to 7. Further, 130 g of sodium sulfate is added underagitation and the resulting solution is dried. Then, the obtained powderis calcined at 1000° C. for two hours. After cooling, the obtainedproduct is washed within warm water under agitation in order to removesulfate and sodium sulfate, and TiO₂ flakes having the rutile typecrystal structure are obtained. The average particle diameter is 45 μm.

The obtained TiO₂ flakes are added into the nitro-cellulose lacquer inan amount of 5% by weight, based on the total weight of the mixture. Theobtained coating composition containing the TiO₂ flakes are coated onpaper and then the luster of the dried coating is observed. The coatingshows a sufficient luster in a somewhat smaller degree than the coatingaccording to example 1.

Comparative Example

Deionized water (600 g) is heated to 95° C. Into this, 200 g of titaniumtetrachloride solution (32.0% concentration) is added over the course oftwo hours while adjusting the pH with sodium hydroxide aqueous solution(32.0% concentration). After adding all of titanium tetra-chloridesolution, the pH is raised to 7. Further, 130 g of sodium sulfate isadded and the resulting solution is stirred and then dried. Then, theresulting powder is calcined at 1000° C. for two hours. After cooling,the obtained product is washed within warm water under agitation inorder to remove chloride and sodium sulfate. TiO₂ particles having therutile type crystal structure are obtained. The obtained particles havean average particle diameter of 6 μm and a fiber-like shape with agranular surface.

The obtained TiO₂ particles according to the comparative example areadded to the nitro-cellulose lacquer in an amount of 5% by weight, basedon the total weight of the coating composition. The obtained coatingcomposition containing the TiO₂ particles is coated onto paper andobserved. There was no specific appearance such as luster.

The single crystal TiO₂ flakes obtained by the production methodaccording to the present invention can be utilized as pigments forpaints, inks, coating compositions, plastics and cosmetics, and as aphoto catalyst, host material for dye sensitized solar cells, UVabsorbent or water repellent.

1.-9. (canceled)
 10. A single crystalline TiO₂ flake having a rutilecrystal structure and having an average particle diameter in the rangeof 10 to 200 μm according to the largest length or width of the flake,and having the shape of a letter “V” or a triangle when viewed at thelargest surface of the flake.
 11. A single crystalline TiO₂ flakeaccording to claim 10, wherein the average particle diameter of the TiO₂flake is at least at least 15 μm and at most 200 μm.
 12. A singlecrystalline TiO₂ flake according to claim 10, having an average particlediameter of 15 to 150 μm according to an equivalent of the correspondingcircle diameter.
 13. A single crystalline TiO₂ flake according to claim10, having and aspect ratio of at least
 5. 14. A composition comprisinga single crystalline TiO₂ flake according to claim 10 which is a paint,ink, coating composition, plastic or cosmetic.
 15. A compositionaccording to claim 14 wherein the single crystalline TiO₂ flakefunctions as white pigment, photo catalyst, host material for dyesensitized solar cells, UV absorbent or water repellent.
 16. A singlecrystalline TiO₂ flake according to claim 13, wherein the TiO₂ flake hasan aspect ratio of at least 7.5.
 17. A single crystalline TiO₂ flakeaccording to claim 10, wherein the TiO₂ flake has an average thicknessof 0.2 μm-1.0 μm.